Food items such as poultry, fresh red meat and cheese, as well as nonfood industrial and retail goods, are packaged by various heat shrink film methods. There are two main categories of heat shrink films--hot-blown shrink film and oriented shrink film. Hot-blown shrink film is made by a hot-blown simple bubble film process and oriented shrink film is made by elaborate processes known as double bubble, tape bubble, trapped bubble or tenter framing. Heat shrink films can be monoaxial or biaxial oriented and are required to possess various other film attributes. In addition to a high shrink response, for successful use in hot-fill or cook-in applications, shrink films must also possess a relatively high softening point.
The shrink packaging method generally involves placing an article(s) into a bag (or sleeve) fabricated from a heat shrink film, then closing or heat sealing the bag, and thereafter exposing the bag to sufficient heat to cause shrinking of the bag and intimate contact between the bag and article. The heat can be provided by conventional heat sources, such as heated air, infrared radiation, hot water, combustion flames, or the like. Heat shrink wrapping of food articles helps preserve freshness, is attractive, hygienic, and allows closer inspection of the quality of the packaged food. Heat shrink wrapping of industrial and retail goods, which is alternatively referred to in the art and herein as industrial and retail bundling, preserves product cleanliness and also is a convenient means of bundling and collating for accounting and transporting purposes.
The biaxial heat-shrink response of an oriented polyolefin film is obtained by initially stretching fabricated film to an extent several times its original dimensions in both the machine and transverse directions to orient the film. The stretching is usually accomplished while the fabricated film is sufficiently soft or molten, although cold drawn shrink films are also known in the art. After the fabricated film is stretched and while still in a stretched condition, the stretch orientation is frozen or set in by quick quenching of the film. Subsequent application of heat will then cause the oriented film to relax and, depending on the actual shrink temperature, the oriented film can return essentially back to its original unstretched dimensions, i.e., to shrink relative to its stretched dimension.
Hence, clearly the orientation window and shrink response of oriented films affected by resin properties and fabrication parameters. The orientation window depends upon the broadness of the resin melting range and, as such, relates directly to the short chain branching distribution of the resin. In general, ethylene alpha-olefin interpolymers having a broad short chain branching distribution and broad melting range (e.g., heterogeneously branched ultra low density polyethylene resins such as ATTANE.TM. resins supplied by The Dow Chemical Company) exhibit a wider orientation window compared to ethylene alpha-olefin interpolymers characterized as having a narrow short chain branching distribution and narrow melting range (e.g., homogeneously branched linear ethylene polymers such as EXCEED.TM. and EXACT.TM. resins supplied by Exxon Chemical Corporation).
Oriented polyolefin film shrinkage depends on shrink tension and film density. Film shrinkage is decreased as the orientation temperature is increased due to lower shrink tension. Film shrinkage is increased at lower density (lower crystallinity) because crystallites provide topological constraints and, as such, hinder free shrinkage. Conversely, for given draw ratio, shrink tension depends on the crystallinity of the resin at the orientation temperature.
While the temperature at which a particular polymer is sufficiently soft or molten is a critical factor in various orientation techniques, in general, such temperatures are ill-defined in the art. Disclosures pertaining to oriented films that disclose various polymer types (which invariably have varying polymer crystallinities and melting points), simply do not define the stretching or orientation temperatures used for the reported comparisons. U.S. Pat. No. 4,863,769 to Lustig et al., WO 95/00333 to Eckstein et al., and WO 94/07954 to Garza et al., the disclosures of which are incorporated herein by reference, are two examples of such disclosures.
The direct effect of density or crystallinity on shrink response and other desired shrink film properties such as, for example, impact resistance, are known, for example, from WO 95/08441, the disclosure of which is incorporated herein by reference. That is, even where the orientation temperature is presumably constant, lower density polymer films will show a higher shrink response and improved impact resistance. However, the effects of density and other resin properties on the orientation temperature is not well-known. In the prior art, there are only general rules of thumb or generalized teachings relating to suitable stretching or orientation conditions. For example, in commercial operations, it is often said that the temperature at which the film is suitably soft or molten is just above its respective glass transition temperature, in the case of amorphous polymers, or below its respective melting point, in the case of semi-crystalline polymers.
While the effects of density and other resin properties on the optimum orientation temperature of polyolefins are generally unknown, it is clear that heterogeneously branched ethylene polymers such as ATTANE.TM. resins and DOWLEX.TM. resin have a relatively broad orientation window (i.e., the temperature range at which the resin can be substantially stretched when molten or softened). It also clear that softening temperatures and other film properties such as, for example, secant modulus, tend to decrease at lower polymer densities. Because of these relationships, films with high shrink responses, wide orientation windows, high modulus and high softening temperatures (i.e., shrink films with balanced properties) are unknown in the prior art. That is, polymer designers invariably have to sacrifice high softening temperatures and high modulus to provide films with high shrink responses and wide orientation windows. The importance of higher modulus pertains to, for example, the need for good machinability during automatic packaging operations and good handling during bag making operations.
An example of teaching that's beyond ordinary rules of thumb (but is nevertheless fairly generalized) is provided by Golike in U.S. Pat. No. 4,597,920, the disclosure of which is incorporated herein by reference. Golike teaches orientation should be carried out at temperatures between the lower and higher melting points of a copolymer of ethylene with at least one C.sub.8 -C.sub.18 .alpha.-olefin. Golike specifically teaches that the temperature differential is at least 10.degree. C., however, Golike also specifically discloses that the full range of the temperature differential may not be practical because, depending on the particular equipment and technique used, tearing of the polymer film may occur at the lower end of the range. At the higher limit of the range, Golike teaches the structural integrity of the polymer film begins to suffer during stretching (and ultimately fails at higher temperatures) because the polymer film then is in a soft, molten condition. See, U.S. Pat. No. 4,597,920, Col. 4, lines 52-68 bridging to Col. 5., lines 1-6. The orientation temperature range defined by Golike (which is based on higher and lower peak melting points) generally applies to polymer blends and heterogeneously branched ethylene/.alpha.-olefin interpolymers, i.e. compositions having two or more DSC melting points, and does not apply at all to homogeneously branched ethylene/.alpha.-olefin interpolymers which have only a single DSC melting point. Golike also indicates that a person of ordinary skill can determine the tear temperature of a particular polymer and discloses that for heterogeneously branched interpolymers having a density of about 0.920 g/ cc, the tear temperature occurs at a temperature above the lower peak melting point. See, U.S. Pat. No. 4,597,920, Col. 7, Example 4. However, Golike does not teach or suggest how a person of ordinary skill in the art of shrink film can optimize the orientation process as to stretching temperature at a given stretching rate and ratio to maximize the shrink response and achieve balanced properties.
Hideo et al. in EP 0359907 A2, the disclosure of which is incorporated herein by reference, teach the film surface temperature at the starting point of stretching should be within the range of from 20.degree. C. to about 30.degree. C. below the melting temperature of the polymer as determined in regards to the main DSC endothermic peak. While such teaching is considered applicable to homogeneously branched ethylene/.alpha.-olefin interpolymers having a single DSC melting peak, the prescribed range is fairly general and broad. Moreover, Hideo et al. do not provide any specific teaching as to the optimum orientation temperature for a particular interpolymer respecting heat shrink response, nor any other desired shrink film property.
WO 95/08441, the disclosure of which is incorporated herein by reference, provides generalized teachings pertaining to homogeneously branched ethylene/cc-olefin interpolymers. In the Examples of this disclosure, several different homogeneously branched substantially linear ethylene/.alpha.-olefin interpolymers were studied and compared to one heterogeneously branched ethylene/ .alpha.-olefin interpolymers. Although the homogeneously branched substantially linear ethylene/.alpha.-olefin interpolymers had densities that varied from about 0.896 to about 0.906 g/cc, all of the interpolymers (including the heterogeneously branched linear ethylene/.alpha.-olefin interpolymer, ATTANE.TM. 4203, supplied by The Dow Chemical Company, which had a density of 0.905 g/cc) were oriented at essentially the same orientation temperatures. Reported results in WO 95/08441 disclose three general findings: (1) at an equivalent polymer density, substantially linear ethylene/.alpha.-olefin interpolymers and heterogeneously branched linear ethylene/cc-olefin interpolymers have essentially equivalent shrink responses (compare Example 21 and Example 39 at pages 15-16), (2) shrink responses increase at lower densities and constant orientation temperatures, and (3) as orientation temperature increases, orientation rates increase. Furthermore, careful study of the Examples and unreported DSC melting point data for the interpolymers reported on in WO 95/08441 indicate for the Examples disclosed in WO 95/08441 that, at a given stretching rate and ratio, there is a preference for orienting multilayer film structures at orientation temperatures above the respective DSC melting point of the polymer employed as the shrink control layer. Moreover, none of the teachings or Examples in WO 95/08441 suggest a shrink film with balanced properties is obtainable.
Other disclosures that set forth orientation information regarding homogeneously branched ethylene polymers yet do not specify orientation conditions relative to respective lowest stretch temperatures, nor teach requirements for balanced shrink film properties include EP 0 600425A1 to Babrowicz et al. and EP 0 587502 A2 to Babrowicz et al., the disclosures of which are incorporated herein by reference.
Accordingly, although there are general rules and general disclosures as to shrink responses and suitable orientation temperatures for biaxially orienting polyolefins, there is no specific information as to optimum orientation conditions as a function of polymer type and, more importantly, there is no specific information as to balanced or optimized shrink responses, wide orientation windows, high modulus and high softening temperatures. As such, it is an object of the present invention to provide an improved shrink film with a maximized shrink response, an increased orientation window and, for a given modulus or polymer density, a relatively high softening temperature . This and other objects will become apparent from the description and various that follow.